Systems for tire pressure detection are known in the art where the tire pressure is determined on the basis of tire pressure measuring modules measuring the tire pressure and emitting a corresponding test value to a receiver installed in the vehicle (also referred to as TPMS, i.e. ‘Tire Pressure Monitoring System’). A module of this type can e.g. be mounted close to the valve in the wheel rim or can be structurally united with the valve. A prior art system founding on pressure measurement uses in each wheel a co-rotating wheel module that is integrated into the valve and measures the tire pressure and the tire temperature by means of corresponding sensors. This data is transmitted in wireless manner to a receiver installed in the vehicle and processed in an electronic evaluating device. The received and processed test values are used either to indicate a pressure value or to produce alarm signals when the tire pressure falls below predetermined threshold values, i.e. when tire pressure loss is detected.
The employment of a system of direct measurement such as TPMS necessitates indicating a tire pressure value, which is independent of ambient conditions, in particular temperature. This is because it is generally felt to be unpleasant when a pressure value shown on the control panel undergoes excessive variations, although e.g. only the driving condition or the outside temperature has changed.
U.S. Pat. No. 4,909,074 discloses a method of detecting, evaluating and displaying a value of a tire pressure, wherein an information signal that represents a detected pressure is compared with a large number of specifically selectable, desired curves of values. These curves of values illustrate pressure ranges for respectively defined values of the ambient temperature and the inner tire temperatures (as well as the ambient pressure, as the case may be) with defined tolerance ranges respectively depending thereon. A corresponding signal is produced when it results from the comparison that the actual tire air pressure is outside the tolerance range. Further, the information signal can be compared with a desired tire pressure that is associated with a predetermined maximum speed. It is then possible to issue a maximum safe speed in dependence on the result of the comparison.
Thus, the ambient temperature is taken into account in this method when detecting, evaluating and displaying a tire pressure or its allowable value. However, this method, exactly as the TPMS method initially referred to, still suffers from the problem that the temperature, which is measured by a sensor e.g. in the valve in a per se known fashion, generally is the wheel rim temperature, yet not (necessarily) the actual temperature of the gas in the tire.
This is because the wheel rim temperature, above all with relatively quick temperature variations, does not correspond to the temperature of the gas in the tire. It may occur, for example, that the brake disc of the motor vehicle and, as a consequence thereof, also the wheel rim heats up relatively quickly, with the result that a temperature gradient develops between tire and wheel rim and the gas temperature is initially essentially lower than indicated by the sensor signal.
Further, it may occur that the ambient temperature changes suddenly and significantly (the motor vehicle is e.g. driven out of a garage). Due to different heat conductions and a differing specific heat of the materials and the related significantly quicker variation of the temperature of the wheel rim compared to the tires mounted, this change may also lead to the said undesirable temperature gradients or to faulty measurements of the gas temperature.
Further, methods for tire pressure loss detection are known in the art that operate without pressure sensors such as the system DDS (Deflation Detection System) of Continental Teves, Frankfurt am Main. A change of the rolling circumference of the tire that is caused by a pressure change is detected and evaluated in these systems. Like in the above-mentioned systems, it may be desirable in these or other systems to indicate a pressure value irrespective of the gas temperature in the tire, on the one hand. On the other hand, there is the problem that a sensor used to detect and compensate this temperature will not indicate the actual gas temperature in the tire due to external influences such as a hot brake disc, but indicates a higher temperature (maybe also a lower temperature under different influences) so that temperature compensation is effected wrongly.